The present invention is directed to energy conversion, particularly the conversion of heat energy into mechanical power, and particularly into usable electrical power.
The generation of electrical power is, for the most part, accomplished by the conversion of heat into mechanical energy, which in turn is utilized for the generation of usable electrical power. The most widely used systems for this purpose involve the use of fossil fuels in the form of natural gas, fuel oil or coal to generate heat of combustion, which is utilized to create steam for driving turbines. The mechanical output of the turbines is employed to drive electrical generators for the production of electrical energy.
One of the seemingly insurmountable problems inherent in this procedure for electrical power generation is the relatively low conversion efficiencies in terms of energy input versus usable electrical output. Inherent in any process for the conversion of heat energy into mechanical output is that the useful output is a function of the temperature (and therefore the enthalpy) of the fluid medium (be it steam or other fluid) at the inlet of the conversion apparatus, minus the temperature of the fluid medium at the outlet side of the conversion apparatus. Efficiencies generally can be optimized by increasing the inlet temperature of the working fluid and/or by decreasing the outlet temperature of the fluid. Maximum inlet temperatures are, however, limited by the ability of the materials to withstand temperatures and pressures. Likewise, temperatures at the outlet side are limited by practical considerations of being able to reject excess heat to the ambient environment.
As a practical matter, electrical generating plants operating purely on the conversion of fossil fuels to electrical energy via a steam process have achieved maximum efficiencies only in the low 40% range, with well over 50% of the heat energy being wasted by rejection to the environment. The use of gas turbines for power generation enables the conversion process to be operated at a higher temperature range, but still involve very low efficiency rate in the conversion of heat energy to usable electrical output. By combining high temperature gas turbine engines with steam-power generation, to achieve a combined cycle, it is possible to extract some of the waste heat from the gas turbine process and employ it usefully in the steam process, increasing overall thermal efficiencies to about 54% in the modern power plants. Even in these "highly efficient" modern combined cycle power plants, however, almost half of the available energy is lost to the ambient by way of rejected heat.
In accordance with one aspect of the present invention, a novel combined energy conversion process is provided, which enables startling and unexpected improvements to be realized in the thermal efficiency of converting heat energy to usable electric power. By way of the system of the invention, thermal efficiencies approaching 70% can be realized.
In accordance with a further objective of the invention, an energy conversion system according to the invention is readily adaptable to relatively small, low power installations, which are highly compatible with the concept of "energy islanding". In this respect, in the generation of electrical power, the primary outputs are electrical power, on the one hand, and rejected heat, on the other hand. It is a relatively simple technological matter to transmit the electrical power over long distances for distribution to consumers wherever located. However, the rejected heat is usefully employed only to a very limited extent because of difficulties in transmitting it over long distances. Through the use of energy islanding concepts, it is possible to deploy relatively small generating installations in a widely distributed manner, locating them close to consumers of heat energy (or in some cases, chilling service). It is a very simple matter to transmit the electrical power away from the consumers of the heat energy, for utilization wherever the electrical energy may be in demand. The energy conversion system of the invention is uniquely advantageous in this respect, both in the ability to install and operate units of small power capacity, and in the flexibility of deriving either heat energy or chilling service, as may be needed. Pursuant to the invention, an energy conversion system is provided which consist of two closed cycle systems, one functioning as a heat engine, to convert heat energy into electrical energy, and the other functioning as a heat pump, driven by a portion of the output of the first system. The two systems, with closed, circulating working fluids, are joined at a common indirect heat exchanger. The heat exchanger advantageously is maintained at cryogenic temperature levels by the heat pump system and constitutes a virtual heat sink for the rejection of heat from the circulating working fluid of the power-generating heat engine. By virtue of the extremely low heat rejection temperature level of the heat engine, the thermal conversion efficiency of the heat engine is extraordinarily high. Although some of the output of the heat engine is extracted and utilized to drive the heat pump system, the overall efficiency of the combined system is nevertheless significantly greater than obtainable with currently utilized systems.
In one particularly desirable form of the invention, the heat engine system is a gas turbine, operating on a closed Brayton cycle, utilizing argon as a working fluid. The heat pump system likewise is in the nature of a gas turbine unit, but is operated in reverse, in that electrical energy from the generating system is employed to drive the turbine, which is used to compress a closed system circulating fluid, (again preferably argon gas). The compressed gas, after rejection of heat of compression, is expanded at the common indirect heat exchanger, providing cryogenic temperature levels for rejection of waste heat from the heat engine.
The use of gas turbine technology is particularly desirable for the purposes of this invention because it is well understood and equipment is readily available. The basic principles of the invention are, however, not limited to the use of gas turbines operating in a closed Brayton cycle. Stirling cycle engines may also be employed to advantage, one engine to convert heat energy to power output, and a second Stirling engine functioning as a heat pump, driven by a portion of the power output of the heat engine. Hybrid modifications of available vehicular power plants can be economically made to derive economical Stirling cycle systems.
Although in principle the teachings of the invention are applicable to large centralized power stations, they are synergistically useful as applied to smaller power generating installations, geographically distributed in closer proximity to consumers of the otherwise largely wasted thermal output, with excess power being transmitted over conventional power lines to a monitored distribution network.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments of the invention, and to the accompanying drawings.